In powder metallurgy (PM), elemental copper powder is oftentimes added to iron powders, along with graphite powder, to cost-effectively improve the mechanical properties of sintered PM steel compacts. Typically, about 1.5 to about 2.5 wt. % of copper is added to the mixture to achieve these mechanical benefits.
Despite copper's advantages, it tends to cause undesirable dimensional growth in the sintered compact. Variation in size between compacted parts results in waste and increased costs. The extent of this distortion is dependent on the amount of elemental copper in the composition and the level of segregation of copper in the PM mixture. Likewise, the addition of graphite, while adding strength to the compacted part, tends to also have a significant effect on the dimensional properties of the sintered compact. Given the dimensional variability that iron-copper-graphite alloys are susceptible to, producing sintered parts having a high degree of dimensional precision is difficult using such a mixture.
FIG. 1 depicts the dimensional change of iron-based alloys including from 0 to about 2 wt. % copper, based on the weight of the alloy, and from 0.6 to about 1 wt. % of graphite, based on the weight of the alloy. As can be understood from FIG. 1, those iron-based alloys comprising about 1 wt. % copper maintained good dimensional control with respect to variations in graphite content. Unfortunately, alloys comprising 1 wt. % of copper are insufficient for most PM applications and are not widely used. Rather, alloys including about 1.5 to about 2.5 wt. %, preferably 2 wt. %, copper are widely used in the industry. Unfortunately, as can be seen from FIG. 1, alloys comprising about 1.5 and about 2 wt. % copper do not have good dimensional control with respect to variations in graphite content.
As such, PM materials that include copper and graphite, while minimizing dimensional changes, are needed.